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Question regarding electroscopes

  1. Jan 31, 2008 #1
    [SOLVED] Question regarding electroscopes

    I was just wondering if the leaves at the bottom of a grounded electroscope will repel when induced by a charged object, regardless of the type of charge.

    I was told that if a positively charged rod was placed near a neutral electroscope, the leaves would move closer to each other. That does not make sense to me because I thought that the electrons would move towards the positive rod while the protons stay where they are (in the leaves) and the like charges would cause the leaves to move farther away from each other.

    It would be greatly appreciated if someone could point me in the right direction. Thank you.
  2. jcsd
  3. Feb 1, 2008 #2


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    When a charged object is brought near the plate at the top of an earthed electroscope it induces an opposite charge on the plate. This excess charge is "held in place" by the proximity of the charged object. The rest of the electroscope need not hold any additional excess charge since the earth can absorb or supply such required charge. So the leaves will be down while the induction is in progress. The excess charge will also be held in place when the earth is removed by the charged object so the leaves will remain down and will only raise when the charged object is removed and the excess charge redistributes throughout the electroscope.

    Last edited: Feb 1, 2008
  4. Feb 1, 2008 #3
    Thank you for the the explanation, but I have another question to ask. What happens to the leaves of a charged electroscope if another charged object is held near the knob? If the electroscope is charged positively and a negatively charged object is held near the plate, then I think the leaves would be down because the electrons would travel towards the leaves, neutralizing the effects of the protons. And if the charged object is positive, then the leaves would raise because the electrons would travel towards the positively charged object, leaving behind the protons which repel each other.
    Sorry for asking so many questions, but I'm just not sure if my train of thought is correct.
    Last edited: Feb 1, 2008
  5. Feb 4, 2008 #4


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    Correct. A negatively charged object will push some excess electrons away from the knob towards the leaves. This will decrease the amount of positive charge on them causing them to go down. A positively charged object will pull some excess electrons up towards the knob thereby increasing the amount of positive charge at the bottom and causing the leaves to raise even further.
  6. Feb 4, 2008 #5
    Thank you very much for your help.
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